/Engine/Source/Runtime/Slate/Public/Framework/Application/SlateApplication.h
Registers a viewport with the Slate application so that specific messages can be routed directly to a viewport This is for all viewports, there can be multiple of these as opposed to the singular "Game Viewport"
void RegisterViewport ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SViewport](API\Runtime\Slate\Widgets\SViewport) > InViewport )